Building insurers have condemned what they call the "utter inadequacy" of government-backed tests used to certify cladding systems containing combustible plastics.

The Association of British Insurers claims to have created a more realistic test which shows fires in real buildings can burn 100C hotter with flames spreading up to four times further than in official tests.

Their research, carried out by the Fire Protection Association (FPA), was commissioned in the wake of the Grenfell Tower fire in June 2017 which spread up an external cladding system and killed 71 people.

According to the FPA, factors "overlooked" in official testing of combustible plastics include test fires not being hot enough and cladding not including gaps and openings.

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They claim 20% of the fuel in a real fire comes from plastics, which they found burns 100C hotter than the all-wood fire used in the official cladding test.

In a test of insulation and cladding that was not sealed the FPA recorded flames six metres above the fire they had lit, while a fire under the same material in a sealed cladding system went out after spreading 1.5 metres.

The findings have been sent to Dame Judith Hackitt who is in the final stages of a government-commissioned review of building regulations.

In a formal submission the ABI says her review "should make recommendations that ensure the use of combustible materials is not permitted, especially in key vulnerable parts of buildings".

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Jonathan O'Neill, managing director of the FPA, said the British Standards Institution (BSI), which oversees testing standards, should look again at the cladding fire test.

"We urge BSI to urgently reconvene the group responsible for this standard to consider the results of this research and to make changes to the standard as required," he said.

"If the [standards] committee view is that the changes are supported by the evidence and are technically feasible, they will work together to amend the standard," they said.

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The government's Ministry of Housing, Communities and Local Government, which commissioned seven of the official fire tests after the Grenfell Tower disaster to assess which cladding systems were safe for tower blocks and which need to be removed or modified, said: "We are committed to constantly improving safety standards."